There is extensive and growing interest in lab-on-a-chip technologies. Such a broad interest in microfluidic technologies extends from their many advantages over traditional laboratory methods, such as the ability to carry out separation and detection with high resolution and sensitivity, need for only very small quantities of sample and reagent, small footprint of the analytical devices these chips contain, low cost of manufacture, and short time of analysis.
With such great promise, a diverse host of platforms used to house the microfluidic channels and devices have emerged. The result has been a plethora of highly specialized micro/nanofluidic systems each incompatible with the others. Optimally, a universal platform could be developed comprised of functional building blocks—each layer performing a specific function—that could be stacked one on top of the other to rapidly create versatile and highly customizable chips for a particular application, leading to a low-cost development cycle since stacked chips can be connected by a network of fluidic and electrical through silicon vias (TSVs). The road to create such a system requires the ability to fabricate micro/nanofluidic channels and devices that can be manufacturably encapsulated in such a way as to provide a fluid-tight seal for the fluidic channels.
The sealing of fluidic chips has generally been accomplished one of the following ways. Polymer Systems—much of the exploratory research in the field of microfluidics has be carried out using poly(dimethylsiloxane) or PDMS, which is a soft elastomer. Among its primary advantages are its ease of use to create structures using soft lithography and its optical transparency. This material provides an early stage development tool where pattern negatives can be quickly replicated by spinning the material onto structures patterned in resist or other materials that are supported by substrate, and then cured, removed, and bonded to a target substrate without the need for downstream processing. See, for example, McDonald et al., “Fabrication of microfluidic systems in poly(dimethylsiloxane),” Electrophoresis, 21(1), pgs. 27-40 (January 2000). The primary disadvantages of this material are i) that it does not have sufficient chemical and thermal stability for some applications, hence ii) it is also not compatible with advanced semiconductor processing techniques that can add further functionality and scaling.
Glass Bonding—the mechanical stability of silicon and glass make them useful where rigid walls are needed to precisely control dimensions that are not accessible to PDMS fluidic structures. Additionally, silicon offers a process advantage. Very commonly a process, such as anodic bonding, is used to seal glass to pieces of silicon or silicon dioxide covered material that have micro/nanofluidic features defined in them without the need for an adhesion layer that can redefine or constrict fluidic features. The glass acts as a ceiling to hermetically encapsulate the micro-mechanical silicon elements. Typically, glass bonding to silicon is achieved through rigorous and labor intensive cleaning and preparation measures. See, for example, Jia et al., “Bonding of Glass microfluidic Chips at Room Temperatures,” Anal. Chem., 76, pgs. 5597-5602 (August 2004). Afterward, pressure must be carefully applied to ensure a good seal without breaking the glass or silicon. Another challenge is that fluidic access holes must be drilled into the glass coverslip to provide an entrance and exit for fluidics flowing into and out of the chip, which is technically challenging on very flat and thin pieces of glass. Finally, long anneals are typically applied to strengthen the glass-silicon or glass-silicon dioxide bond. Glass to glass bonding has also been demonstrated. Nonetheless, none of these approaches provide a clear path to making manufacturable chips at a large scale.
Therefore, improved techniques for sealing of fluidic chips that are fully compatible with three-dimensional integrated silicon technology or chip stacking would be desirable.